EP patent number EP2312360 describes a carbon fibre intervention cable rod with three parallel and mutually insulated electrical conductors wherein the bundle of said three insulated electrical conductors are pultruded in a process adding a structural carbon fibre layer to make a rod which may be injected into a production well. The carbon fibres are parallel in order to maximize tensile strength of the rod. A disadvantage with such a structural carbon fibre layer is that it may disrupt radially and break partially or snap off entirely, such as when pushed with a force of about 5000 N or when subject to a sudden pressure drop.
Pultruded composite rods with a mantle of unidirectional carbon fibre around a core constituted by two parallel electrical conductors, as illustrated in FIGS. 3 and 7 or three electrical conductors are known in the field of petroleum intervention such as in the above EP2312360. When used in a petroleum well, high-pressure intrusion of fluids may incur disintegration of the unidirectional carbon fibres when the pressure is abruptly relieved, particularly when being hauled out when the rod cable leaves the grease stuffing box at the top of the wellhead where the pressure gradient is at its highest. The rod's unidirectional fibres may disrupt laterally and easily disintegrates further, and becomes longitudinally soft and completely useless for injection, when pushing the rod into the well, so-called “rodding”, at the very same instant, even for minor outbreaks. In case of such a disruption, the entire rod on the reel has to be replaced. If the rod breaks in the well the portion remaining inside the well must be fished. Fishing a highly split broken end of a carbon fibre rod is a difficult task because it splits into an irregular bundle of separate strands of different thicknesses.
An electrical cable core of a carbon fibre intervention rod with twisted insulated electrical conductors of the background art is illustrated in FIG. 3. The carbon fibre mantle portion with unidirectional carbon fibres is omitted, but the entire cross-section of such a rod with the unidirectional fibre mantle is shown in FIG. 7. The cable core of the background art composite rod is provided with two closely arranged insulated conductors, said two conductors having a minimum thickness of insulation so as for avoiding local electrical short-circuit between the two conductors. The cross-section areas of each of the two conductors are equal.
Coaxial signal cables are often provided with a thin insulated centric signal wire and a rather rugged coaxial screen of far higher cross-section area, of which the role of the coaxial screen is purely for the role of screening the centric signal wire from external electromagnetic signals, and of which the centric signal wire shall have optimal signal transmission properties.